Sputtering system with spirally coiled coolant coil

ABSTRACT

The anode and the workpieces of a thin film sputtering system are cooled by coolant that circulates through spiral tubular coils located below the anode and into chambers located within the anode. The anode is rotatable, and the cooling coils expand and contract to permit partial rotation of the anode. The two cathodes of the sputtering system are cooled by coolant that circulates through chambers within the cathodes. The cathodes are designed to minimize contamination from residue coolant leaking into the gastight enclosure of the sputtering system while the cathodes are being removed. The gastight enclosure is connected to a vacuum pump through a port. A throttle plate, located within the enclosure over the port, aids the vacuum pump to efficiently regulate the gas pressure in the enclosure during a sputtering run.

Feb. 27, 1973 B-.-.-J. ROBIISON ET AL 3,718,572

SPUTTERING SYSTEM WITH SPIRALLY COILED COOLANT COIL Filed Oct. 10, 19712 Sheets-Sheet 1 Coolant Coolant To RF Power Impedance Matching Network16 pp y Cathode (Target) 12 Rotatable Anode 6 piral Tubular CoilsCoolant To Vacuum Pumping System B. J.- ROBISON ET AL 3,718,572

Feb. 27, 1973 SPUTTERING SYSTEM WITH SPIRALLY COILED COOL-ANT COIL-Filed Oct. 10, 1971 2 Sheets-Sheet 2 pzjooo uzE =9.

United States Patent 3,718,572 SPU'ITERING SYSTEM WITH SPIRALLY COILEDCOOLANT COIL Billie J. Robison, Palo Alto, and Richard A. D. Lucie,

Sunnyvale, Calif., assignors to Hewlett-Packard Company, Palo Alto,Calif.

Filed Oct. 7, 1971, Ser. No. 187,264 Int. Cl. C23c 15/00 US. Cl. 204-298Claims ABSTRACT OF THE DISCLOSURE The anode and the workpieces of a thinfilm sputtering system are cooled by coolant that circulates throughspiral tubular coils located below the anode and into chambers locatedwithin the anode. The anode is rotatable, and the cooling coils expandand contract to permit partial rotation of the anode. The two cathodesof the sputtering system are cooled by coolant that circulates throughchambers within the cathodes. The cathodes are designed to minimizecontamination from residue coolant leaking into the gastight enclosureof the sputtering system while the cathodes are being removed. Thegastight enclosure is connected to a vacuum pump through a port. Athrottle plate, located within the enclosure over the port, aids thevacuum pump to efficiently regulate the gas pressure in the enclosureduring a sputtering run.

BACKGROUND OF THE INVENTION In exacting uses of sputtering systems, thepresence of contaminants in the environment of the sputtering process isundesirable. During sputtering, the target of highly pure material isbombarded by gas ions that cause atoms of the target material to sputteroff the target and deposit on substrate wafers, called workpieces.Contaminants in the process hinder the :bonding of the deposits andcreate impurities in the deposited layer, or film. The contaminants maydirectly aifect the workpieces by adhering to the surfaces of theworkpieces, or they may indirectly affect them by contaminating thesputtering apparatus and the target material.

Contamination of the gastight enclosure of the sputtering system mayoccur in many ways. Leaks in the enclosure seals may allow the entry ofwater or air particles into the process. These particles may causeoxidation of. the deposited material, thereby-changing the properties ofthe film and ruining the process. Similarly, the target may becomeoxidized, especially if an oxygen-sensitive material such as molybdenumis used. Both air and water often carry contaminants such as oil or dustthat may collect on the workpieces or the target if they enter during.the sputtering process.

Out-gas from materials during sputtering is also a source ofcontamination. This may result from the sublimation of the material orfrom the release of absorbed or adsorbed gases by the material.Elastomer seals are examples of components that exhibit excessiveout-gassing at low pressures.

An additional problem with vacuum seals arises when a rotating shaft orconduit enters the enclosure. It is more diflicult to seal around arotating element because the rotation eases the entry of contaminants bypropelling the particle along the turning shaft through the vacuum seal.

One major source of contamination in a sputtering process is the coolingnetwork. Both the workpieces and the target are cooled by a circulatingcoolant system to maintain proper operating temperatures. If the anodeis rotatable and used as a workpiece holder, the cooling system for theworkpieces, which operates by cooling the anode, must also rotate. Thisnecesitates either the entry of a rotating conduit into the vacuumenclosure or the presence of a joint between a rotatable and astationary conduit inside the enclosure itself. As mentioned above, aseal for a rotating shaft increases contamination problems. The internaljoint requires a watertight seal that prevents the escape of coolant inthe chamber. The present state of the art has not developed seals foreither of the above cases that totally prevent contamination.

Another source of contamination is the cathode assembly. Since it issometimes desirable to change the target material, most cathodes arecomposed of a target support and a separable target. If the cathodecontains chambers for liquid coolant, the junction between the targetand the support is usually sealed by an O ring that encircles thecathode. This 0 ring increases the probability of (rut-gassing andleakage. In addition whenever the target is detached from the cathodeassembly in the gas-tight enclosure, any residue coolant remaining inthe cathode may fall into the enclosure.

The quality of sputtering depends upon other criteria in addition to theabsence of contaminants. Constant pressure and temperature are two suchfactors. During sputtering, the enclosure gas pressure must bemaintained at a predetermined pressure. To ease the work load put on thevacuum pump, the cross section of the port leading into the enclosure isreduced in area, usually by the adjustment of a butterfly valve in thepipe connecting the vacuum pump with the enclosure. This is an imprecisetechnique because the butterfly valve cannot be repeatedly set to thesame position. As a result, establishing the proper enclosure pressureis sometimes a lengthy process involving continual readjustment of thebutterfly valve and the gas inlet valve that regulates gas flow into theenclosure.

SUMMARY OF THE INVENTION This invention relates to a thin filmsputtering system which includes three separate improvements. Thecooling system for the rotatable anode includes spiral, tubular coilslocated inside the gastight enclosure. These coils expand and contract,like a watchspring, as the anode turns. Because the cooling coils do notrotate, a joint between a stationary and a rotary conduit isunnecessary. Thus, there is no need for the water-tight seal thatnormally encloses such a joint. Moreover, the cooling system no longerneeds a seal that can accommodate a rotary shaft because the conduit isstationary at the entry and exit points to the enclosure. With theelimination of these seals, contamination is reduced.

If the target is to be removed, the cathode assembly, consisting of boththe target and the target support, can be easily removed as a unit,thereby permitting access to the target outside the vacuum enclosure.The only connections in the cathode cooling system are external to theenclosure. Consequently, disconnection of the cooling system prior tocathode removal confines the remaining coolant within the cathode. Whenthe cathode is removed from the enclosure, the coolant residue is alsoremoved. Thus there is little chance of contamination during thisprocess. The problem of out-gassing from the cathode assembly isconsiderably reduced because this invention does not require an O ringseal between the target and the target support.

The present invention diminishes the problem of establishing the desiredgas pressure inside the gastight enclosure by replacing the butterflyvalve with a throttle plate having a central hole. The area of the holeequals the optimum size of the orifice necessary for the vacuum pump tomaintain the desired gas pressure during a sputtering run. The plate canbe raised and lowered over the port that connects the vacuum pump to theenclosure. Prior to sputtering, the plate is lowered over port, and theport size is decreased to the size of the central hole. Since this plateis used for every sputtering run, the port is restricted to exactly thesame area for every run. Thus, establishing the enclosure gas pressurebecomes a quick and easy process when this invention is used.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows a cutaway side view of animproved sputtering system according to the preferred embodiment of thisinvention.

FIG. 2 shows a partially cutaway top view taken along the line 22 of thesputtering system of FIG. 1.

FIG. 3 shows a detailed cutaway side view of the cathode assemblies ofthe sputtering system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, acylindrical gastight enclosure 2 is connected to a vacuum pumping systemthrough the vacuum port 4. An anode 6 that is capable of rotating on abearing assembly 8 is driven by a cable drive system 10. The anode actsas a holder for the workpiece wafers 11 that will be coated by thetarget material during the sputtering process. Each of the two cathodes12 is composed of a target and a target support. One cathode has a goldtarget; the other, molybdenum. These materials are used to sputtermolybdenum-gold contacts onto semiconductor wafers. Other materials canbe used by replacing the targets.

The temperature of the workpieces is controlled by cooling their holder,the anode, by coolant that circulates through chambers within the anode.The coolant, usually water, flows into the gastight enclosure throughconduit 13 that passes through a feedthrough 14. Then it flows throughthe spiral tubular coils 15 and into the anode cooling chambers. Itreturns through a parallel conduit over the same path.

The cathodes are cooled by circulating coolant through chambers withinthe cathodes. An RF power supply is connected to the cathodes by animpedance matching network 16. The impedance matching network comprisesinductors and capacitors interconnected to match the impedance of thepower supply with the impedance of the ionized plasma .within theenclosure. Impedance matching is performed to minimize high frequencypower reflections from the sputtering System back to the power supply.The ground return of the power supply is connected to the anode throughthe enclosure chassis. Alternatively, a D.C. power supply may be used toreplace the RF sup ply and the impedance matching network.

A plasma confining shield 17 that is at the same electrical potential asthe anode restricts the plasma and the sputtered particles to theimmediate area of the workpieces. Each target in the enclosure isprovided with such a shield.

Before the sputtering can begin, the proper atmosphere of inert gas mustbe established within the gastight enclosure. Before pumpdown, thethrottle plate 19 is raised to its upper position above the vacuum port4 by turning the knob on the bellows-sealed rotary feedthrough 23, adevice well known in the art. The rotary tfeedthrough is linked to thethrottle plate so the plate can be raised or lowered by turning the knobon the rotary feedthrough. With the plate in its upper position, port 4is unrestricted and the vacuum pumping system can evacuate theenclosure.

After the enclosure has been evacuated, the throttle plate is loweredover the vacuum port, closing the port except for the small central hole18 in the plate. Gas is supplied to the enclosure through the gas inlet20, and the vacuum pump maintains the proper gas pressure inside theenclosure during the sputtering run.

It is desirable to have maximum gas flow from inlet 20,

through the enclosure, and out port 4 during the sputtering run becausethe gas flow helps sweep any gaseous contaminants that may be present inthe enclosure out the port. However, a high gas fiow increases thenumber of gas molecules that the vacuum pump must evacuate. If the gasflow exceeds the capacity of the vacuum pump, the pump may cease tooperate and will begin to contaminate the gastight enclosure. Mostvacuum pumps include a heavy liquid bath, such as an oil bath, that isused to pull the atmosphere from the enclosure. If the maximum flow rateof the pump is exceeded, molecules of oil will travel from the pump,through the port, into the enclosure. This will contaminate thesputtering apparatus with oil, a highly undesirable contaminant.

The orifice 18 in the throttle plate is designed to provide the highestflow that will not cause oil contamination of the enclosure by thevacuum pump. The flow through the orifice when the throtle plate islowered over the port is given by the following equation:

Q=fiow in standard cubic centimeters per second AP=(desired gas pressureof enclosure)-(normal pressure of vacuum pump) A=area of orificeT=temperature in K.

M=molecular weight of the gas This equation is derived from thewell-known Efiusion Law:

F=3.638A /T/M where:

F=maximum flow rate in liters per second =area of orifice T=temperaturein K.

M=molecular weight of the gas The flow, Q, is determined from thespecifications of the vacuum pump and from the amount of oilcontaminants tolerated in the enclosure during a sputtering run. For theenclosure used in FIG. 1, it has been experimentally determined that aflow equal to approximately 50% of the maximum flow specified by thevacuum pump manufacturer produces optimal conditions for a puresputtering run. Consequently, for a vacuum pump listed at maximum Qequal to 1 cubic centimeter per second and having a normal pressure of.2 micron, the area of orifice 18 according to the above equation isapproximately 5.5 square centimeters for an enclosure containing argongas maintained at a sputtering pressure of 7 microns. This area has beenexperimentally validated for the sputtering system in FIG. 1.

The area of central hole 18 may change for different sputtering systemsand for the degree of purity required in the sputtered contacts.However, the use of an orifice provides the advantage of preciserepeatability of the flow rate for every sputtering run made with thesame sputtering system. Once the optimal size of the central hole isdetermined, the port is always restricted to exactly that size duringsputtering by lowering the throttle plate over the port. This exactnesscannot be repeatedly achieved with conventional methods.

Prior to the evacuation of the enclosure, the workpieces are placed onthe anode surfacein an area under one cathode. To establish uniformsputtering, the wafers must be contained within the boundary establishedby the plasma-confining shield. The anode is rotated by the cable drivesystem 10 until the workpieces are positioned under the cathode thatwill not be used as the target. Then the enclosure is evacuated and thegas is inserted. The RF supply is connected to the cathode that will beused as the target during this sputtering run. The supply is thenenergized, causing the target material to be deposited onto the anodewithin the confines of the plasma shield. This initial sputter is usedto remove contamination from the surface of the target. After asufiicient layer has been removed from the target, the anode is rotated180 so the workpieces are positioned directly under the target. Now thetarget material is deposited onto the workpieces until the desired layerhas been deposited. If it is desired to deposit material from the othertarget onto the workpieces, the process is repeated with the secondtarget energized.

Throughout the sputtering process, both the anode and the cathodecooling systems are in operation. When the anode rotates from oneposition to the other, the coolant conduit must rotate with it. Becausethe conduit is coiled in a spiral (FIG. 2), it can move with therotating substrate holder. Like a watchspring, the coil expands when theanode moves in one direction and contracts when it moves in the other.As a result, the conduit connected to the underside of the anode canmove with the rotating anode while the conduit at the entry feed through14- remains stationary. Thus there is no need for a joint betweenrotating and stationary conduits, nor for a vacuum seal to accommodate arotating conduit.

Note that the anode only moves through 180 of rotation. If greaterrotation is desired, the cooling conduit can be modified to accommodatethis. The number of turns in the coil can be increased, or the conduitcan be constructed from a more elastic material.

Referring now to FIG. 3, each cathode 12 is composed of a target 21 or22 and a target support 24-. The gold target 21 is brazed to its holderwhile the molybdenum target 22 is attached by screws 36. Each targetsupport contains cooling chambers 32. Coolant, supplied through theexternal coolant conduits 34, cools the targets by circulating throughthe chambers. The gold target 21 makes direct thermal contact with itstarget support while the molybdenum target 22 makes thermal contactthrough copper pads 30. These assemblies eliminate the need for an Oring between the target and the target support.

To change the target material, the cathode must be removed irom thevacuum enclosure. First, the joints 40 to the external conduits 34 aredisconnected while the cathode remains in position. This allows anycoolant remaining in the external conduit to fall outside the enclosureor into the cathode, but not into the enclosure itself. Next, thecathode is disconnected from the RF source by disconnecting theelectrical connector 28. Then the clamping nut 26 is unscrewed andremoved from the cathode shaft. This releases the cathode from the discseal and the ceramic insulator 27. Now the entire cathode assembly,including the target and target support, can be lowered into theenclosure and removed. \Any coolant remaining in the cooling chamber canbe emptied outside the enclosure. Once the cathode is outside, thetarget can be removed and a new one installed. As a result of thisinvention, the target material can be quickly changed with little chanceof contamination to the sputtering system.

What we claim is:

1. A sputtering system comprising:

an enclosure for maintaining a gas at a selected pressure;

a cathode supported within the enclosure;

an anode supported for angular motion within the enclosure, said anodeincluding a chamber for coolant; means coupled to the anode forimparting partial rotational motion thereto; and

at least one spirally coiled conduit positioned within the enclosure andcoupled to the anode chamber for supplying coolant thereto, the spiralcoils of said spirally coiled conduit expanding when the anode turns inone direction and contracting when the anode turns in the oppositedirection.

2. A sputtering system as in claim 1 wherein:

said cathode comprises a target and a target support forming a coolantchamber; and

said sputtering system includes clamping means connected to the cathodefor attaching the cathode to the enclosure, whereby the clamping meanscan be disconnected to separate the cathode from the enclosure.

3. A sputtering system as in claim 2 including:

a port in the enclosure to permit evacuation of the enclosure;

a plate positioned within the enclosure above the port, said plate beingsupported for movement between a raised position and a lowered positionat which it restricts the port by a selected amount; and

means coupled to the plate for raising and lowering the plate.

4. A sputtering system as in claim 3 wherein:

said plate has a central hole smaller than the port and in the loweredposition rests on the surface of the enclosure to restrict the port tothe size of the central hole; and

said clamping means electrically insulates the cathode from theenclosure.

5. A sputtering system as in claim 4 including:

a vacuum pumping system connected to the port for evacuating theenclosure; and

means for supplying gas to the enclosure;

whereby said central hole enables the vacuum pumping system to maintaina desired gas pressure within the enclosure without the entry ofcontaminants from the vacuum pumping system through the port into theenclosure.

6. A sputtering system as in claim 1 wherein:

said cathode has a target and a target support forming a coolant chamberand is detachably supported within the enclosure so that the cathode canbe disconnected from the enclosure.

7. A sputtering system as in claim 6 wherein said target support is partof the target.

8. A sputtering system as in claim 6 wherein said target is fixedlyattached to the target support.

9. A sputtering system as in claim 6 wherein said target is attached tothe target support.

10. A sputtering system as in claim 6 including means for electricallyinsulating the cathode from the enclosure.

References Cited UNITED STATES PATENTS 3,296,115 1/ 1967 Laegreid et al204298 3,528,906 9/1970 Cash et al 204298 3,558,467 1/ 1971 Jackson204-298 3,617,459 11/1971 Logan 204298 3,630,881 12/1971 Lester et al204-298 TA-HSUNG TUNG, Primary Examiner S. S. KANTER, Assistant Examiner

